Trained male cyclists (N=40) ingested quercetin (Q, N=20) (1000 mg/day) or placebo (P, N=20) supplements under randomized, double blinded methods for 3 wks prior to and during a 3-d period in which subjects cycled for 3 h/day at ~57% Watts_max. Blood samples were collected before and after each exercise session and assayed for plasma IL-6, IL-10, IL-1ra, IL-8, TNF-, and MCP-1, and leukocyte IL-10, IL-8, and IL-1ra mRNA. Muscle biopsies were obtained before and after the first and third exercise sessions and assayed for NFB, and cyclooxygenase-2 (COX-2), IL-6, IL-8, IL-1 and TNF-mRNA. Post-exercise increases in plasma cytokines did not differ between groups, but the pattern of change over the 3-d exercise period tended to be lower in Q versus P for IL-8 and TNF- (P=0.094 for both). mRNA increased significantly post-exercise for each cytokine measured in blood leukocyte and muscle samples. Leukocyte IL-8 and IL-10 mRNA was significantly reduced in Q versus P (interaction, P=0.019 and 0.012, respectively) with no other leukocyte or muscle mRNA group differences. Muscle NFB did not increase post-exercise and did not differ between Q and P. Muscle COX-2 mRNA increased significantly post-exercise, but did not differ between Q and P. In summary, one gram/day quercetin by cyclists over 24-days diminished post-exercise expression of leukocyte IL-8 and IL-10 mRNA, indicating that elevated plasma quercetin exerted effects within the blood compartment. Quercetin did not, however, influence any of the muscle measures including NFB content, cytokine mRNA, or COX-2 mRNA expression across 3-days of intensified exercise.